A nanofiber is an ultrafine fiber having an extremely small diameter of about 1 urn or less, and is applied in diverse fields such as a medical material, a filter, micro electro mechanical systems (MEMS), a nano device, etc. The nanofiber has a very large surface area per unit mass, is flexible, and has plenty of micro spaces and large numbers of fibers present per unit area, and may be blended with other materials and may have good dispersibility with respect to external stress.
As one method of preparing the nanofiber, an electrospinning method is known. An electrospinning apparatus used in the electrospinning method includes a spinning tip for discharging a solution, a high voltage apparatus and a collector for collecting the nanofiber. A high voltage is applied to the spinning tip to charge droplets discharged from the spinning tip, and a stream is spurt from the droplets by electrostatic repulsion to form the nanofiber in the collector. In addition, the nanofiber may be prepared using microfluidic technique. The nanofiber having a core shell structure may be prepared using an apparatus composed of an injection tube and a collection tube, making middle fluid and outer fluid different and then, pushing with an external pressure.
Since the nanofiber has a very large surface area, the diversity of the function of the surface may be maximized. For example, the nanofiber may be used for preparing a functional nano device by forming an elastic electrode through the formation of a semiconductor nanowire on the nanofiber. The functional nano device may allow the formation of enable stretchable electronics, wearable devices, etc.
Meanwhile, a photocatalyst decomposing various non-degradable organic pollutants by absorbing light to produce a strong oxidant such as hydroxyl radicals is used in photocatalyst technique. The photocatalyst used may include a semiconductor oxide such as TiO2, ZnO, WO3 and SnO2, and the TiO2 photocatalyst is known to have the best efficiency among them. Studies on using the photocatalyst in water treatment or an air purification process have been conducted a lot; however have not been commercialized because of defects such as the recovery of a powder, etc.
Recently, a composite carbon nanofiber having photocatalytic activity is prepared by obtaining a carbon fiber precursor through electrospinning, coating photocatalyst sol and carbonizing to form a composite of a carbon nanofiber having effective adsorptive power with a photocatalyst capable of providing high decomposition efficiency. However, the coating process of the photocatalyst is necessary, and SiO2 is added to the photocatalyst sol to be coated to prevent the deterioration of the efficiency of the photocatalyst due to the phase change of the photocatalyst during performing a carbonizing process, thereby inducing the decrease of the specific surface area of the composite.
In addition, in a common method, a spinning solution obtained by adding a titanium dioxide photocatalyst has been used for conducting the electrospinning for preparing a carbon nanofiber without a coating process. A composite nanofiber including titanium dioxide could be obtained through the method, and a carbon nanofiber containing titanium dioxide having an anatase crystalline phase and high photocatalytic activity has been prepared through a post oxidation process for forming the anatase crystalline phase. A carbon nanofiber structure containing the titanium dioxide may be prepared without conducting a coating process, however, many of titanium dioxide particles having the photocatalytic activity are buried in the carbon nanofiber, and the photocatalytic activity could not be exhibited.